Hydrogen-induced delayed fracture under mode II loading
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I.
INTRODUCTION
H Y D R O G E N induced cracking (HIC) and stress corrosion cracking (SCC) of high-strength steel under Mode I loading have been investigated extensively, i-4 In addition, HIC and SCC can also occur under Mode III loading, s'6 though there is no hydrostatic stress, i.e., O'h = O. McGuinn et al. found SCC of high-strength steel in 3.5 pct NaC1 aqueous solution under Mode II loading. 7 They proposed, however, that the nature of SCC should be HIC but wrongly thought O'h = 0 in a Mode II specimen. Magdowski 8 and Nichols 9 pointed out that O'h =~ 0 in the Mode II specimen and it had a maximum value when 0 = - 1 8 0 deg or 0 = - 1 0 0 deg, according to linear elastic fracture mechanics or elastic-plastic fracture mechanics, respectively, where 0 was an angle inclined to the original crack plane. Can HIC and SCC of high-strength steel occur under Mode II loading? Where do HIC and SCC initiate and what is the direction crack growth? The present work is an attempt to answer the questions. Since the fatigue crack initiating from the notch tip very often deviates from the notch direction under Mode II alternative loading, it is difficult to obtain an ideal Mode II precracked specimen. Therefore a notched specimen had to be used in the present work. The stress field is known for an infinitely large specimen under Mode II loading,I~ but for a real Mode II specimen there must be a bending moment at the notch tip. It is evident that the stress field in the real Mode II specimen will differ from that in the infinitely large specimen. Therefore the stress field in the real Mode II TONG-YI ZHANG, Lecturer, WU-YANG CHU, ASsociate Professor, and CHI-MEI HSIAO, Professor, are with the Department of Metal Physics, Beijing University of Iron and Steel Technology, China. YING LI, formerly Student at Beijing University of Iron and Steel Technology, is now with the Second Heavy Machine Factory, China. Manuscript submitted April 3, 1985. METALLURGICAL TRANSACTIONS A
specimen should be calculated in order for us to be able to analyze reasonably well the experimental results of HIC and SCC. To investigate the mechanism of SCC under Mode II loading, the initiation site and direction of SCC have been measured and compared with those of HIC. Besides, the normalized threshold stress intensity factors of SCC and HIC under Mode II or Mode I loading were measured and a theoretical explanation has been proposed. II.
EXPERIMENTAL PROCEDURES
The 34CrNi3Mo steel used in the present studies had a composition of 0.33C, 0.25Si, 0.67Mn, 0.93Cr, 2.80Ni, 0.34Mo in weight percent and balance Fe. All specimens were austenitized at 900 ~ and oil-quenched, then tempered at 260 ~ for 2 hours. The tensile strength of the specimens was 1700 MPa. The dimensions of the specimens are shown in Figure 1. The general gripping arrangement for shear testing of the Mode II specimen is illustrated in Figure 2. Tests under
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Fig. 1 - - Specimen dimension (ram); the arrows indexed
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